A typical plasma etching apparatus comprises a reactor in which there is a chamber through which reactive gas or gases flow. Within the chamber, the gases are ionized into a plasma, typically by radio frequency energy. The highly reactive ions of the plasma are able to react with material, such as the dielectric between interconnects or a polymer mask on a surface of a semiconductor wafer during it being processed into Integrated Circuits (IC's). Prior to etching, the wafer is placed in the chamber and held in proper position by a chuck or holder which exposes a top surface of the wafer to the plasma.
In semiconductor processing, the etch or deposition rate uniformity across the wafer during each process directly affects the device yield. This has become one of the main qualifying requirements for a process reactor and hence is considered a very important parameter during its design and development. With each increase in the size of wafer diameter, the problem of ensuring uniformity of each batch of integrated circuits becomes more difficult. For instance, with the increase from 200 mm to 300 mm in wafer size and smaller circuit size per wafer, the edge exclusion shrinks to, for example, 2 mm. Thus maintaining a uniform etch rate, profile, and critical dimensions all the way out to 2 mm from the edge of the wafer has become very important.
In a plasma etch reactor, the uniformity of etch parameters' (etch rate, profile, CD, etc.) is affected by several parameters. Maintaining uniform plasma discharge and hence plasma chemistry above the wafer has become very critical to improve the uniformity. Many attempts have been conceived to improve the uniformity of the wafer by manipulating the gas flow injection through a showerhead, modifying the design of the showerhead, and placing edge rings around the wafer.
One problem in a capacitively-coupled etching reactor having electrodes of different sizes is the lack of uniform RF coupling especially around the edge of a wafer. FIG. 1 illustrates a conventional capacitively-coupled plasma processing chamber 100, representing an exemplary plasma processing chamber of the types typically employed to etch a substrate. Referring now to FIG. 1, a chuck 102, representing the workpiece holder on which a substrate, such as a wafer 104, is positioned during etching. Chuck 102 may be implemented by any suitable chucking technique, e.g., electrostatic, mechanical, clamping, vacuum, or the like. During etching, chuck 102 is typically supplied with dual RF frequencies (a low frequency and high frequency), for example 2 MHz and 27 MHz, simultaneously during etching by a dual frequency source 106.
An upper electrode 108 is located above wafer 104. Upper electrode 108 is grounded. FIG. 1 illustrates an etching reactor where the surface area of upper electrode 108 is larger than the surface area of chuck 102 and wafer 104. During etching, plasma 110 is formed from etchant source gas supplied via a gas line 112 and pumped out through an exhaust line 114.
When RF power is supplied to chuck 102 from RF power source 106, equipotential field lines are set up over wafer 104. The equipotential field lines are the electric field lines across the plasma sheath that is between wafer 104 and the plasma 110.
During plasma processing, the positive ions accelerate across the equipotential field lines to impinge on the surface of wafer 104, thereby providing the desired etch effect, such as improving etch directionality. Due to the geometry of the upper electrode 108 and the chuck 102, the field lines may not be uniform across the wafer surface and may vary significantly at the edge of the wafer 104. Accordingly, a focus ring 118 is typically provided to improve process uniformity across the entire wafer surface.
An electrically conductive shield 120 substantially encircles the focus ring 118. Electrically conductive shield 120 is configured to be grounded within the plasma processing chamber. Shield 120 prevents the presence of unwanted equipotential field lines outside of focus ring 118.
Confinement rings 116 may be placed between upper electrode 108 and a bottom electrode, such as the chuck 102 in FIG. 2. In general, confinement rings 116 help confine the etching plasma 110 to the region above the wafer 104 to improve process control and to ensure repeatability. Confinement rings 116 are positioned at a predetermined radial distance from wafer 104 posing a physical barrier for further plasma expansion. However, because the diameter of confinement rings 116 cannot be changed, the diameter of plasma 110 and thus its cross-section is nearly a fixed quantity for all processes. Thus, the active electrode area ratio, which is defined as the surface area of the grounded electrode divided by the surface area of the RF electrode, is static for a plasma reactor having confinement rings statically positioned.
Accordingly, a need exists for a method and apparatus for confining plasma with an adjustable electrode area ratio. A primary purpose of the present invention is to solve these needs and provide further, related advantages.